I believe that you will have to flash the CSC that corresponds to the one in the USA or in Europe. A CSC usually indicates the region of the firmware. That's why Viber, although you have the phone in English, will display other elements in the Vietnamese. Firmware flashing could be problematic because it will wipe all your data from the phone, therefore create a backup.
You can download the firmware files for your model using the Samsung Updates website. Select the model of your phone from the drop down menu under the Devices on the left.
Also, change the region and the language of your phone to be in complete English(US).